Production of corrosion-resistant coatings on aluminum and aluminum alloys



Patented Nov. 21, 1933 I UNITED STATES PRODUCTION OF CORROSION-RESISTANT COATINGS N ALUMINUM AND ALUMINUM ALLOYS Alexander Jenny, Berlin-Charlottenburg, and Nikolai Budiloil', Berlin-Friedenau, Germany, assignors to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a

corporation of Germany No Drawing. Application December 13, 1932, Serial No. 646,978, and in Germany December t Claims.

The present invention relates to the production of corrosion-resistant coatings on aluminum and aluminum alloys by passage of an alternating current in acid baths, especially saturated chromic acid baths. In this process an object of aluminum or an aluminum alloy, already provided with a protective coating, is used as often as desired as counter-electrode in producing a protective coating on a second object of aluminum or of an alu- 1 minum alloy. When using relatively low concentrations of chromic acid, for instance 0,05% or 3% chromic acid solutions there is the possibility of producing uniformly colored protective coatings in many cases but the use of saturated solu- 1 tions is more advantageous.

In testing this process by using different aluminum alloys, there was found that in some cases irregularities in the color of the protective coatings occurred, especially in the production 2 of black coatings on aluminum alloys, known under the name Lautal and Duralumin". To avoid this drawback, the invention provides a method, in which a saturated chromic acid solution is used for obtaining a coating of oxide on the counter-electrode by passage of an alternating current and advantageously also for obtaining a protective coating .on untreated objects oi! aluminum and aluminum alloys. The protective coatings can be produced by employing a bath temperature ranging between about and 60 C. and by employing voltages oi. alternating current between about 40 and 60 volts for a period of about 20 minutes.

A chromic acid solution, saturated at room temperature (20 C.) which contained about 102 g of solid chromic acid '(CrOa) in 100 com of water and had a specific weight of about 1.71 was used for the production of oxide containing coatings on two plates of Duralumin. The temperature of the solution was maintained at 50 C. and an alternating voltage of 50 volts was employed. After about 20 minutes one of the two" plates had a hard, nearly colorless, glass-like coating and on the other plate was deposited a pearl-grey coating.

Then the so treated plates were used as countective coatings on Lautal was similar to that of tor. sot-1)- coating of the second counter-electrode, howeve. remained practically unchanged.

It was observed that increases or decreases voltage of not more than about 10 volts and variations in bath temperature of not more t about 1%" C. had no essential influence on qualities of the coatings, especially on the c and the protective action. The coating proces could be extended to 30 minutes when a voltage of 40 volts. The best coatings were produced with a temperature of C. and an alternating voltage of 50 volts in a coating period of 20 minutes. At volts unsatisfactory coatings were obtained.

Almost the same results were found to be produced with plates of Lautal. The same chromic acid solutions and substantially the same bath temperatures and bath voltages could be used for producing good coatings. The color of the prothe above mentioned coatings on Duralumin; only on the white surface of Lautal the coatings had a somewhat gloomy aspect.

On objects of pure aluminum there are always deposited by the above described process dark brownish glass-like coatings on one electrode and bright grey coatings on the other electrode.

In order to dye the coatings produced on aluminum or on an aluminum alloy it is advantageous to dip the coatings immediately after their production and after washing with water into suitable organic dye solutions or to paint or flow such solutions upon the described coatings.

It will be understood that, in executing the new method, not only chromic acid solutions saturated at room temperature (20 C.) can be used but also chromic acid solutions saturated at the working temperature (40450" C.). The formation of the saturated chromic acid solutions advantageously takes place by dissolving crystalline chromicacid (CrOa) in water which is heated to a higher temperature than the desired temperature of saturation and by cooling the resulting solution to the temperature 01' saturation afterwards. Such "a quantity of crystalline chromic acid must be employed that a portion of the chromic acid remains on the bottom of the solution vessel in solid state.

.comprises immersing such articles in a saturated solution of chromic acid and passing alternating no current between said articles through said solution.

2. The process of claim 1 wherein a previously coated article is employed as a counter electrode 5 to the article which is being coated.

3. The process of claim 1 wherein the temperature of the bath is maintained between about 40 to C. and an alternating current voltage of about 40to 60 volts is employed.

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